The present invention relates to a method of and an apparatus for injection molding objects or workpieces formed at least of two different layers.
The production of multi-layer workpieces or objects, with the layers being formed of different material is well known. E.g., German Patent DE 23 42 794 C3 discloses a method of manufacturing of multi-layer workpieces from thermoplastic materials, which have a foamed core and a non-foamed outer layer or shell. According to the method disclosed in DE 23 42 794 C3, first, a portion of a non-foamed plastic material and then, simultaneously, a foam agent-containing plastic material, together with the remaining portion of the non-foamed material, are injected through a central nozzle and a ring nozzle, which surrounds the central nozzle, into a mold, with the non-foamed plastic material being injected through the central nozzle and the foaming agent-containing plastic material being injected through the ring nozzle. The workpieces formed of a foaming agent-containing plastic material have an advantage over the workpieces formed of a non-foamed, compact material, which consist in that, on one hand, they can be produced irrespective of their wall thickness and, on the other hand, they are much lighter, without the stability or rigidness of workpieces being affected.
German Patent DE 30 25 597 C2 discloses an injection head for injection molding of multi-layer workpieces from two different plastic materials which are fed from two separate injection cylinders into the injection head. The injection head includes a central nozzle extending in an injection direction and a ring nozzle surrounding the central nozzle, with the mouth of the central nozzle, when viewed in the injection direction, being located in front of the mouth of the ring nozzle through which another plastic material is fed. A common, central cylindrical valve body extends in the injection head in the injection direction and is displaced, in the injection direction, by an external drive so that its front end portion is displaced into the mouth of the ring nozzle.
The common valve body serves for closing and opening of the ring nozzle mouth as well as for feeding of a plastic material to the central nozzle.
To solve certain technical problems and, if necessary, to achieve a certain decorative effect, the objects or workpieces are imparted a certain external appearance or particular functional characteristics by appropriate finishing of the outer surface of the objects or workpieces. To this end, the objects or workpieces are often covered with lacquer, which requires a separate operational process and a separate apparatus for effecting lacquering.
German Application DE 24 49 758A discloses a method for producing workpieces from a foaming agent-containing plastic material and which permits to obtain an outer surface having an improved quality. According to the method described in DE 24 49 758A, the plastified plastic material is injected from the nozzle of an extruder into a mold cavity of an appropriate mold against an air or gas cushion, which is retained in the mold cavity under an adjustable pressure and which is forced out of the mold cavity under the counter pressure of the plastic material. However, the objects or workpieces, which are produced by this method, still require finishing lacquering that necessitate an additional flat coating and grinding process.
Finally, German Patent DE 23 46 135 C2 discloses a process of continuous manufacturing of multi-layer workpieces pieces from a plastic material and having a foamed core and an non-foamed outer coating. According to the method described in this patent, the non-foamed plastic material is delivered through a ring nozzle, and the foaming agent-containing plastic material is delivered through a nozzle arranged in the ring nozzle. According to the described process, the foaming agent-containing plastic material is injected into the mold together with a core of a non-foamed plastic material. The process, disclosed in DE 23 46 135 C2, permits to insure, without an increase in the amount of plastic material forming the non-foamer outer coating, that the outer coating has the same thickness over the entire outer surface of the workpiece and does not have any interruption, is continuous over the entire outer surface.
The object of the invention is a method of and an apparatus for producing multi-layer objects or workpieces which permit to simplify the production of such objects and workpieces.
Another object of the invention is a method of and apparatus for producing multi-layer objects or workpieces which permit to substantially improve the quality of the outer surface.